CREATE [AGGREGATE] FUNCTION [IF NOT EXISTS] function_name
    RETURNS {STRING|INTEGER|REAL|DECIMAL}
    SONAME shared_library_name

This statement loads the loadable function named function_name. (CREATE FUNCTION is also used to created stored functions; see Section 15.1.17, “CREATE PROCEDURE and CREATE FUNCTION Statements”.)

A loadable function is a way to extend MySQL with a new function that works like a native (built-in) MySQL function such as ABS() or CONCAT(). See Adding a Loadable Function.

function_name is the name that should be used in SQL statements to invoke the function. The RETURNS clause indicates the type of the function's return value. DECIMAL is a legal value after RETURNS, but currently DECIMAL functions return string values and should be written like STRING functions.

IF NOT EXISTS prevents an error from occurring if there already exists a loadable function with the same name. It does not prevent an error from occurring if there already exists a built-in function having the same name. IF NOT EXISTS is supported for CREATE FUNCTION statements beginning with MySQL 8.0.29. See also Function Name Resolution.

The AGGREGATE keyword, if given, signifies that the function is an aggregate (group) function. An aggregate function works exactly like a native MySQL aggregate function such as SUM() or COUNT().

shared_library_name is the base name of the shared library file containing the code that implements the function. The file must be located in the plugin directory. This directory is given by the value of the plugin_dir system variable. For more information, see Section 7.7.1, “Installing and Uninstalling Loadable Functions”.

CREATE FUNCTION requires the INSERT privilege for the mysql system schema because it adds a row to the mysql.func system table to register the function.

CREATE FUNCTION also adds the function to the Performance Schema user_defined_functions table that provides runtime information about installed loadable functions. See Section 29.12.21.10, “The user_defined_functions Table”.

During the normal startup sequence, the server loads functions registered in the mysql.func table. If the server is started with the --skip-grant-tables option, functions registered in the table are not loaded and are unavailable.